S. Boubanga-Tombet, S. Chan, A. Satou, V. Ryzhii, and T. Otsuji
We report within picosecond time scale, fast relaxation and relatively slow recombination dynamics of photogenerated electrons/holes in an exfoliated graphene under infrared pulse excitation. We conduct time domain spectroscopic studies using an optical pump/terahertz probe and optical probe technique and show that graphene sheet amplifies an incoming terahertz field. The graphene emission spectral dependency on laser pumping intensity shows a threshold like behavior, testifying the occurrence of the negative conductivity and the population inversion. The phase behavoir of the measured Terahertz electric field also show clear Lorentzian-like normal dispersion around the gain peak, testifying the occurrence of amplification that can be attributed to stimulated emission of photo-carriers in the inverted states. The emission spectra also show a clear narrowing at longer terahertz probe delay time giving a clear evidence of the quasi-fermi energy moving closer to the equilibrium at this longer terahertz probe delay time.
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